EP1629303A1

EP1629303A1 - Method for scanning and analysing a three-dimensional structure

Info

Publication number: EP1629303A1
Application number: EP04732683A
Authority: EP
Inventors: Jacques Dory
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-05-22
Filing date: 2004-05-13
Publication date: 2006-03-01
Also published as: US20070197912A1; FR2855271A1; WO2004104632A1; FR2855271B1

Abstract

Method for scanning and analysing a three-dimensional structure by suitably processing signals representing waves, particularly ultrasonic waves reflected or transmitted by said three-dimensional structure, which processing involves restoring or analysing the three-dimensional structure on the basis of data read out of a field memory. The method comprises calculating, for each structural point, the field memory positions containing the signals detected by the detection elements, corresponding to the waves reflected or transmitted by said point, and data in the field memory is read out by a decoding curve. The method includes detecting the sign of the detected signals, detecting a useful zone of said decoding curve in which the signals at the same sign, and calculating the position and the point being analysed on the basis of an integration of the amplitude of the signals detected in the useful zone of the decoding curve.

Description

PROCESS FOR THE EXPLORATION AND ANALYSIS OF A VOLUME STRUCTURE.

The present invention relates to a method for exploring and analyzing a volume structure by appropriate processing of signals representative of waves, in particular ultrasonic waves reflected or transmitted by this volume structure.

It applies in particular, but not exclusively, to the production of devices such as ultrasound scanners, non-destructive testing devices for objects, sonars or even radars.

Conventional devices of this kind usually involve transmission means which emit an incident wave in the medium to be examined and reception means possibly using all or part of the transmission means which receive the waves reflected by the obstacles encountered by the incident wave. Means are further provided for processing the signals received by the reception means and presenting them in a form usable by the user, for example in the form of an image making it possible to locate the position of the obstacles generating reflections of the incident wave.

The most common method is to use impulse waves in a process of transmitting a pulse of ultrasonic waves in a given direction, detecting the return of echoes, measuring the time between transmission and reception, and deduct the distance, given the speed of propagation of the ultrasonic wave, and therefore the position of the obstacle which generated each echo. This process is then repeated in different directions, according to a predetermined scanning law. It then becomes possible to produce images highlighting the obstacles detected by the echoes, the position of which is known.

In order to avoid the drawbacks of devices using a sequential processing mode (line by line exploration), it has been proposed to emit in the volume structure to be explored a substantially planar wave, of relatively large section generated by a probe made up of a network comprising a plurality of transmission / reception elements of small dimensions, preferably less than the ultrasonic wavelength, so as to have a very wide radiation diagram, these elements being attacked simultaneously in parallel. On reception, each transmission / reception element operates independently and therefore receives separately the waves reflected by the obstacles intercepting the beam of ultrasonic waves which is in its reception area. After digitization, the information delivered by these transmission / reception elements (reflected wave field) is stored in memories whose reading is carried out in the opposite direction to writing.

US-A-4,817,434 describes a device comprising an address generator per receiver element of the probe, which provides the address to be read in a field memory corresponding to the image point to be reconstructed. However, this device only allows a relatively low image reconstruction rate.

With the aim of eliminating this drawback, the Applicant has based itself on the observation that in a process such as that previously described, each point of the object to be explored gives rise to a wave which is stored at addresses of one field memory distributed as an arc of a pseudo-hyperbolic curve, the characteristics of which depend on the position of the point relative to the probe and the radiation pattern of each element (this pseudo-hyperbola is theoretically reduced to two asymptotes for the points located against the probe).

He developed a process comprising the following steps:

- the emission into said structure of an incident wave,

the reception of the waves reflected or transmitted by the obstacles encountered by the incident wave inside said structure, by a plurality of detection elements independent of each other,

the storage, after digitization, of the signals delivered by the detection elements in a field memory comprising a respective line per detection element, and

- the reconstruction and / or analysis of the volume structure from the information read in the field memory, in which are calculated for each point of the structure, the positions of the field memory containing the signals detected by the elements of detection, corresponding to the waves reflected or transmitted by this point, these positions being calculated using an addressing law whose parameters depend on the position of this point relative to the detection elements, and in which for each point , the lines of the field memory are read at the respective positions calculated beforehand for this point and stored in address memories respectively associated with said lines of the field memory.

A calculation is then applied to the information read for this point in order to obtain a result representative of the importance of the wave reflected or transmitted by this point, during this calculation, all the lines of the field memory being read in parallel for each point at the positions indicated for this point respectively by the associated addressing memories, the calculation of the result then being applied to all the values read in the field memory, this result then being processed or stored in a specific memory.

This process described in patents EP 0 825 453 B 1 and EP 0 872 742 B 1 filed in the name of the Applicant, makes it possible to obtain remarkable performances in terms:

- examination speed and therefore authorizes the analysis of large parts,

- spatial resolution taking into account the large radiation aperture, - reproducibility since the ultrasonic field is emitted by a plane wave.

However, experience shows that in certain applications, constraints limit the performance of the process, in particular, the speed of treatment.

The very high spatial resolution obtained with said method leads to very fine images and if one wants to preserve the fineness of the details, and in particular to measure with precision the amplitude of the reflected signals, the analysis frame must be very tight, and as a consequence, the number of points to be calculated is high, which slows down the processing speed excessively. In other words, the maximum amplitude of the echo may not be detected in the analysis frame if it is between two analysis steps; this error can be prohibitive if it proves necessary to measure the importance of the obstacle with great precision. One can be led, depending on the requested precision, to use a smaller analysis step, therefore a greater number of calculated points, which increases the calculation time, and decreases the speed of control.

It turns out that in the process described in patents EP 0 825 453 B 1 and EP 0 872 742 B 1 filed in the name of the Applicant, the analysis step is independent of the number of electronic channels; it is desirable that the analysis step is a multiple or an integer sub-multiple of the step between detection elements; thus, in the case of a probe comprising 32 detection elements, spaced 0.8 mm apart, associated with 32 electronic channels, the analysis step may be equal to 1.6 mm, 3.2 mm, 4.8 mm, 6.4 mm, etc ...

As an example, we can consider the case of probing a metal plate 120 mm thick by a probe of 64 elements with a pitch of 1 mm and a sampling frequency of the detected signal of 60 MHz.

The speed of sound in the material to be probed is 6000 m / s; the journey time, back and forth, is 40 μs.

Knowing that the step of analysis of the detected signal is fixed to the sampling period and taking into account the number of horizontal lines equivalent to the number of elements of the probe, the number of points analyzed is 40 x 60 64, or 153 600.

The total processing time is 153,600 / 60, i.e. 2,560 μs, i.e. a rate of 390 Hz.

It turns out that the lateral resolution at 3 dB of the process, at shallow depth, is of the order of 0.25 mm and that the analysis step adopted in said example of

I mm, can cause errors greater than 10 dB on the amplitude measured; this difference is unacceptable in most cases.

It is therefore necessary to reduce the analysis step by a factor of 4, the processing rate is below 100 Hz; this rate may become incompatible with the speed of control required for large parts. The invention therefore more particularly aims to eliminate these drawbacks by means of a method making it possible to increase the processing speed without sacrificing other performance, by modifying the processing parameters as a function of the situation of the area treated in the memory of field and optimize the speed and / or accuracy of this processing.

1

Optimizing the resolution and processing time is based on decoding the information read from the field memory; thus, the so-called "decoding" curve is the curve for reading the values read from the field memory; the positions of the field memory are calculated using an addressing law, the parameters of which depend on the position of the point to be analyzed relative to the detection elements.

The curve, called "registered", corresponds to the signals detected by the detection elements which correspond to the waves reflected or transmitted by the point or the obstacle to be analyzed.

The present invention is based on the observation that, in a process such as that described above, when the decoding curve does not exactly coincide with the curve written in the field memory, the decoding curve can successively detect positive signals and negative.

Consequently, it proposes a method for the exploration and analysis of a volume structure, from information read in the field memory by a decoding curve, this method comprising the detection of the sign of the detected signals, the detection of a useful area of said decoding curve in which the signals have the same sign and calculating the position of the point analyzed from an integration of the amplitudes of the signals detected in said useful area of said decoding curve.

More specifically, this process may include: - detection of positive and negative signals,

- the detection of the center of said decoding curve,

- memorizing the sign of the signals at the center of the decoding curve,

- the detection of a useful area of the decoding curve in which the signals have the same sign,

- the summation of the signals of the same sign in the useful area of the decoding curve,

the division of said sum by the number of signals analyzed with the same sign or by a predetermined constant value corresponding to the total opening of the decoding curve,

- the memorization of the result obtained for the analyzed point of the volume structure.

Optionally, the method according to the invention may implement, from the information read in an analysis frame and during the storage phase in the field memory:

- the use of a variable horizontal step depending on the analysis depth, and / or

- the modification of the horizontal pitch in the vicinity of the obstacle, and / or - the processing of the signals detected prior to the storage phase.

An embodiment of the method according to the invention will be described below, by way of nonlimiting example, with reference to the associated drawings in which:

FIG. 1 represents the signals detected on the decoding curve and the content of the field memory on either side of the decoding curve, in the case of the coincidence of the decoding curve with the written curve: FIG. 2 represents the signals detected on the decoding curve and the content of the field memory on either side of the decoding curve, in the case of the non-coincidence of the decoding curve with the written curve;

FIG. 3 represents the result of the decoding with the taking into account of the positive and negative signals detected on the decoding curve;

FIG. 4 represents the result of the decoding with the taking into account only of the positive signals detected on the decoding curve;

FIG. 5 represents a theoretical curve of the image of an obstacle in an analysis frame without any correction device;

FIG. 6 represents the envelope of a digitized signal as a function of time, read along a stored field line, before processing;

FIG. 7 represents the envelope of a digitized signal as a function of. time, read along a stored field line, after processing;

FIG. 8 represents a device for processing the digitized signal, read along a stored field line, the results of which are illustrated by FIGS. 6 and 7.

In a first example illustrated by FIGS. 1 to 4, the step of optimizing the spatial resolution, on the basis of the information read from the field memory, comprises the evaluation of the horizontal difference between the decoding curve and the curve entered, and correcting the amplitude of the signals measured as a function of this difference; thus, when the decoding curve does not exactly coincide with the registered curve, the decoding curve can detect successively positive and negative signals, the sum of these so-called positive and negative signals tending towards 0.

Conversely, when the decoding curve exactly coincides with the written curve, the decoding curve detects signals of the same polarity.

Thus, in the example shown in FIG. 1, the content of the field memory is represented by the amplitudes of the detected and stored signals Mi l, M12, M13, M14; the decoding curve CD1 almost exactly coincides with a written curve represented by the detected and stored signals Ml 3 in the field memory; thus, the amplitudes of the signals read from the field memory, represented by M10 are all of the same sign.

Conversely, in the example shown in FIG. 2, the content of the field memory is represented by the amplitudes of the detected and stored signals M21, M22, M23, M24; the decoding curve CD2 does not coincide with any written curve represented by the signals detected and stored in the field memory; the decoding curve is off-center and successively crosses several written curves; thus, the amplitudes of the signals read in the field memory, represented by M20 are sometimes positive, sometimes negative.

Let S + be the number of positive values, S- the number of negative values and SM the total number of positive and negative values on a decoding curve; therefore SM = (S +) + (S-).

Assuming (theoretical case) that all the positive and negative values are of the same amplitude A, the amplitude detected, after summation, is: A0 = SM x A. In the case where the decoding curve does not coincide with the registered curve, the detected amplitude, after summation, becomes: A0 = A x (S +) - A x (S-); which can be written: A0 = A x K, with K = SM - 2 x (S-)

The K factor depends on the difference between the points of opposite signs; it will be equal to SM when all the values have the same sign, and equal to zero when there are as many positive values as negative values.

It should be noted that the factor K is not constant because the values entered vary as a function of their horizontal position on the curve entered; indeed the directivity of the reflecting obstacles causes a maximum of the detected signal in the line of sight of the obstacle, and consequently a weaker detected signal on the periphery around the line of sight.

Thus, the factor K must be replaced by a function f (k); this function can be determined theoretically or experimentally by moving the probe in front of a known obstacle and by measuring for each position of said probe, the amplitude of the reflected signal and the corresponding value of the factor K.

In the example shown in FIG. 3, the coincidence between the decoding curve CD3 and the written curves M31, M32, M33, M34, is not achieved; indeed the signals M30 change sign by deviating from the center of the decoding curve, and the number of signals of the same sign starting from the center is all the smaller the greater the offset, so that the sum of the amplitudes decreases very quickly.

The exact value of the amplitude A can then be obtained simply by dividing A0 by K. This method has the advantage of being able to be carried out in a very simple way, since it suffices to make additions on the sign bit of the stored signal . A very similar process, based on the same principle, but the implementation of which is slightly more complex, can be used to effect this correction and also reduce the amplitude variations as a function of the position of the decoding curves with respect to the written curves.

Indeed, if we add only the signals of the same sign around the central part, and if we divide this sum by the number of signals of the same sign around this central part, we obtain a value very close to S3, if not equal to that of the amplitude detected in the event of perfect coincidence between the decoding curve and the written curve.

This process can be carried out in a sequential manner, by memorizing the sign of the signal at the center of the decoding curve, then by carrying out an analysis on either side of the center, which analysis is stopped in the event of detection of change of sign of the detected signal; thus, the analysis consists in carrying out the sum of the detected signals divided by the number of analyzed signals of the same sign; this value is memorized for the point considered; the operation is then carried out in the same way for the other points to be analyzed, this processing can also be carried out in parallel by logic circuits.

Furthermore, this method excessively amplifies signals corresponding to noise or to signals reflected by obstacles located far from the point to be analyzed, and the resulting image risks being confused.

To avoid this drawback, it is considered that below a certain number of signals of the same sign detected from the center, it is no longer a fault, but a background noise, and the sum of the these signals are then no longer divided by their number, but by a constant value, in general that corresponding to the total opening of the decoding curve. In the example shown in FIG. 4, the coincidence between the decoding curve CD4 and the written curves M41, M42, M43, M44, is not achieved; in the present case, only the central values of the same sign have been taken into account; the resulting curve S4 has an amplitude close to that of the resulting curve S3 and remains constant along a greater horizontal displacement on either side of the center of the decoding curve. Thus, the precision on the amplitude of the detected signal is greater while adopting a larger analysis step.

The various processes mentioned above contribute to the optimization of the spatial resolution, starting from the information read in the field memory; the possible association of processes allowing the increase in the speed of processing of the signals in the analysis frame contributes to the improvement of the performance of the process described in the patents EP 0 825 453 B 1 and EP 0 872 742 B 1 filed on behalf of the Applicant, in particular in the analysis of large parts at high speed.

In this example, according to the invention, the step of optimizing the processing time, from the information read in the analysis frame and during the storage phase in the field memory, firstly comprises l use of a variable horizontal step depending on the analysis depth.

Indeed, the horizontal resolution is a function of the depth of the obstacle; for example, a resolution of 0.25mm to 3mm deep will become 1mm to 30mm deep.

It is therefore possible to adopt a variable horizontal pitch as a function of the depth, said pitch being defined by the acoustic resolution at this depth; thus, knowing the depth of the points to be analyzed, the step analysis will be memorized beforehand and will allow a saving in analysis time of the neighboring structure from 2 to 3.

The step of optimizing the processing time, on the basis of the information read in the analysis frame and during the storage phase in the field memory, secondly comprises modifying the horizontal step in the vicinity of the obstacle.

Indeed, the detection of all the signals whose amplitude is greater than the noise, with a significant analysis step, allows the approximate location of these said signals; finer detection is then carried out in the zone containing each of these said signals.

In the example shown in FIG. 5, a signal S represents a theoretical curve of the image of an obstacle in an analysis frame. Said signal S is characterized by a maximum amplitude V _M and a noise level with maximum amplitude V _B , said theoretical curve being obtained with an analysis step close to zero.

Let V _{A be} the analysis level, lower than the amplitude V _M , considered to be the detection threshold of the signals to be analyzed; a detection error less than Δ = V _M - V _A , imposes an analysis step less than P _l5 defined by the intersection of the curve S and the analysis level V _A.

In the same way, to detect signals of amplitude greater than the noise level, the analysis step will be significantly greater, equivalent to P ₂ , defined by the intersection of the curve S and the noise level of amplitude maximum V _B.

Thus, the detection of the signals will be carried out with an analysis step P ₂ , defined as being the step corresponding to the detection threshold situated above the noise level, making it possible to search for signals whose amplitude is greater than the analysis level V _A.

When an obstacle is detected at a certain abscissa x _p , the analysis of this obstacle consists in resuming the detection of said obstacle, starting from the abscissa x _p - P ₂ with an analysis step Pi. Said analysis is finished in the vicinity of the obstacle, when the detected signal is less than the noise level V _B and the abscissa greater than x _p .

It is therefore conceivable to adopt a variable horizontal pitch depending on the obstacles encountered, said pitch being relatively large, allowing high rates; thus, having detected an obstacle to be analyzed, the analysis step will be reduced in the detection zone of said obstacle; the processing time is practically not affected by the analysis with reduced step compared to the time saving brought by a high initial step.

The step of optimizing the processing time, from the information read in the analysis frame and during the storage phase in the field memory, thirdly comprises the processing of the detected signals.

Indeed, in each field line of the field memory are stored signals reflected or transmitted by an obstacle, in sampled form; thus, the amplitude of the samples represents the envelope of said detected signals; the sampling frequency is higher than the frequency of the detected signals so as to detect the extremes of said signals.

In the example shown in FIG. 6, the sampling period of the detected signal is approximately ten times shorter than the period of said detected signal. The processing of the sampled signal consists in detecting the extremes of said sampled signal, in storing the samples of corresponding amplitude during a half period of the detected signal, and in storing in the field line of the field memory, not all of the samples of said sampled signal, but only the amplitude samples corresponding to the extremes.

In the example shown in Figure 7, the sampled signal consists of the extremes of the original signal, shown in Figure 6; thus, the analysis step of the processed signal can reach the half period of the original signal, while retaining the required precision over the amplitude of the detected signals.

The aforementioned processing can be carried out in software form or in hardware form; in the example shown in FIG. 8, the processing is carried out by material means.

A clock block H delivers a clock signal S _H which is applied on the one hand to a shift register block RD with two stages, and on the other hand to a main memory block M ₂ ; the original signal S, sampled at the frequency of said clock signal S _H , is applied on the one hand to the input of the shift register block RD, and on the other hand to the input of a buffer memory Mi.

The shift register block RD delivers signals S _N and S _{N +} ι corresponding to two successive samples of said signal S; the above signals S _N and S _N + ι are applied to the two inputs of a comparator C whose two outputs switch from state 1 to state 0 depending on whether the sample N is larger or smaller than the sample N + 1; the switch 1 to 0 corresponds to an extremum of the signal S. - o -

Said switching controls a detection circuit D which controls the storage of the signal S in said buffer memory Mj. The output of the memory Mi is then applied to the input of the main memory M ₂ .

Thus, the main memory M ₂ contains the values of the extremes at each half period of the original signal S.

All of the above-mentioned processes for optimizing the spatial resolution, from the information read from the field memory, and for optimizing the processing time of the detected signals can be applied separately or in a combined manner; they thus contribute to making it possible to increase the speed of processing of the detected signals while maintaining a very high spatial resolution, and to allow a three-dimensional analysis of the volume structures at high rate.

Claims

claims

1. Method for exploring and analyzing a volume structure by an appropriate processing of signals representative of waves, in particular ultrasonic waves reflected or transmitted by this volume structure, said processing consisting in reconstructing or analyzing the structure volume from the information read in a field memory, in which are calculated for each structure point, the positions of the field memory containing the signals detected by the detection elements, corresponding to the waves reflected or transmitted by this point, which information in the field memory is read by a decoding curve, characterized in that it comprises the detection of the sign of the detected signals, the detection of a useful area of said decoding curve in which the signals are of the same sign and the calculation of the position of the point analyzed from an integration of the amplitudes of the signals detected in said useful area of said decoding curve.

2. Method according to claim 1, characterized in that it implements: - the detection of positive and negative signals,

- the detection of the center of said decoding curve,

- memorizing the sign of said signals at the center of the decoding curve,

- the summation of said signals of the same sign in the useful area of the decoding curve,

- dividing said sum by the number of signals analyzed with the same sign or by a predetermined constant value corresponding to the total opening of the decoding curve, - the memorization of the result obtained for the analyzed point of the volume structure.

3. Method according to claim 1, characterized in that it comprises, from the information read in an analysis frame and during the memorization phase in the field memory:

- the use of a variable horizontal step depending on the depth, and / or

- the modification of the horizontal pitch in the vicinity of the detected obstacle, and / or

- the processing of the signals detected prior to the storage phase.

4. Method according to claim 3, characterized in that the use of a variable horizontal pitch as a function of the depth comprises measuring the spatial resolution as a function of the depth and storing said resolution.

5. Method according to claim 3, characterized in that the modification of the horizontal pitch in the vicinity of the obstacle comprises a detection of all the signals whose amplitude is greater than the noise, a localization of said detected signals, and a finer detection in the area containing each of said detected signals.

6. Method according to claim 3, characterized in that the processing of the signals detected prior to the storage phase comprises the detection of the extremes of said sampled signals, the storage of samples of corresponding amplitude during a half period of said detected signals, and the storage in the field line of the field memory of the amplitude samples corresponding to the extremums.